1. Field of the Invention
The present invention relates to an attitude detection system for an artificial satellite and, more particularly, to an attitude detection system for detecting the attitude angle of an artificial satellite in a ground station, which is capable of accurately detecting fluctuation of the attitude angle of the artificial satellite over a broad frequency band. The present invention also relates to a method for detecting the attitude angle of the artificial satellite.
2. Description of the Related Art
The attitude of an artificial satellite residing in the space is generally observed from a ground station. FIG. 1 shows an example of the conventional attitude detection system, which includes an attitude-angle calibration sensor 11 for sampling the attitude angle of the artificial satellite at a low frequency to generate an attitude-angle calibration signal 14, an angular-velocity sensor 12 for detecting the angular velocity of the artificial satellite to generate an angular-velocity signal 15 and a sequential Kalman filter 13 for estimating the attitude, i.e., the attitude angle of the artificial satellite by integrating the angular-velocity signal 15 with respect to time while calibrating the integrated data based on the attitude-angle calibration signal 14 at a specified time interval.
Examples of the angular-velocity sensor 12 include a gyroscope, and examples of the attitude-angle calibration sensor 11 include a start tracker. In the sequential Kalman filter 13, the noise characteristics of the attitude-angle calibration sensor 11 and the angular-velocity sensor 12, which are mounted on the artificial satellite, are modeled by using a probability model technique, thereby estimating and removing the noise included in the angular-velocity signal 15 and the attitude-angle calibration signal 14.
The attitude detection system shown in FIG. 1 has a relatively simple structure, and is originally developed as an on-board processing system, i.e., a real-time processing system on the artificial satellite. However, since the ground station can also extract time-series data of the angular-velocity signal 15 and the attitude-angle calibration signal 14 from the telemetry data received by the ground station, the attitude detection system of FIG. 1 is generally and widely used as an on-board processing system as well as a ground processing system.
In the conventional attitude detection system of FIG. 1, although the attitude-angle calibration sensor 11 generally has a higher accuracy compared to the angular-velocity sensor 12, the attitude-angle calibration sensor 11 has a longer measurement cycle which is, for example, more than 10 times longer compared to the measurement cycle of the angular-velocity sensor 12. Accordingly, even if the measurement cycle of the angular-velocity sensor 12 may be significantly improved, the frequency band of the final attitude-angle signal 16 obtained thereby is relatively limited due to the waste time caused by the characteristics of the attitude-angle calibration sensor 11.
It may be considered that the angular-velocity sensor 12 alone is used for obtaining the attitude-angle signal 16 to improve the measurement cycle. However, in this case, there arises a problem that the noise involved in the angular-velocity signal 15 largely affects and degrades the accuracy of the calculated attitude-angle signal 16, although it is possible to detect the fluctuation of the attitude angle of the artificial satellite itself in a higher frequency range.
JP Application 2000-265553 proposes an attitude detection system for an artificial satellite which can solve the above problem in the conventional technique. The proposed system includes an on-board high-frequency attitude-angle sensor, in addition to the attitude-angle calibration sensor 11 and the angular-velocity sensor 12 shown in FIG. 1, thereby generating a broad-band attitude-angle signal.
FIG. 2 shows the proposed system, which includes a telemetry data memory 220 for storing the telemetry data received from the artificial satellite, a first data extractor 201 for extracting attitude-angle calibration data 209 as time-series data from the telemetry data memory 220, a second data extractor 202 for extracting angular-velocity data 210 as time-series data from the telemetry data memory 220, and a third data extractor 205 for extracting high-frequency attitude-angle data as time series data from the telemetry data memory 220.
An angular displacement sensor using a liquid is used as the on-board high-frequency attitude-angle sensor 205, whereby the attitude angle of the artificial satellite can be detected at a higher frequency compared to the angular-velocity data 210. The high-frequency attitude-angle signal, stored in the telemetry data memory 220, is extracted by the third data extractor 205 as a high-frequency attitude-angle signal 213.
The sequential Kalman filter 203 generates an attitude-angle signal 211 based on the attitude-angle calibration signal 209 and the angular-velocity signal 210 extracted by the first data extractor 201 and the second data extractor 202, respectively, from the telemetry data memory 220. The attitude-angle signal 211 generated by the sequential Kalman filter 203 is passed by a low-pass filter 204, interpolated in an interpolator 207, and then delivered to an attitude data adder 208 as a low-frequency interpolated signal 215.
The high-frequency attitude-angle signal 213 extracted by the third data extractor 205 is passed by a band-pass-filter 206 and then delivered to the attitude data adder 208 as a high-frequency attitude signal 214. The attitude data adder 208 adds both the low-frequency interpolated signal 215 and the high-frequency attitude signal 214 together to generate a high-accuracy broad-band attitude-angle signal 216.
In the proposed system of FIG. 2, as described above, the low-frequency attitude signal 212 obtained by the sequential Kalman filter 203 and the low-pass filter 204 is interpolated in the interpolator 207, and then added to the high-frequency attitude signal 214 in the attitude data adder 208 to obtain the high-accuracy attitude signal 216.
In the above operation of the sequential Kalman filter 203, the angular-velocity signal 210 is sampled at a specified time interval corresponding to the measurement interval of the angular-velocity sensor, and integrated with respect to time while being calibrated based on the attitude-angle calibration signal 209. In general, a shorter step interval for the integration provides a higher accuracy. However, in the proposed system, the step interval in the integration is determined by the frequency, or the measurement cycle, of the angular-velocity sensor which has a relatively limited performance as to the measurement cycle, and thus an accurate broad-band attitude-angle signal by the system is difficult to expect.
In view of the above problems in the conventional attitude detection system and the proposed attitude detection system proposed in JP Application 2000-265553, it is an object of the present invention to provide an attitude detection system for an artificial satellite, which is capable of detecting a broad-band attitude-angle signal for the artificial satellite with improved accuracy.
It is another object of the present invention to provide a method for detecting a broad-band attitude-angle signal for the artificial satellite with improved accuracy.
The present invention provides an attitude detection system for an artificial satellite including a telemetry data memory for storing telemetry data received from the artificial satellite, a first data extractor for extracting attitude-angle calibration data from the telemetry data memory as time-series data, a second data extractor for extracting angular-velocity data from the telemetry data memory as time-series data, an interpolator for interpolating the angular-velocity data to generate interpolated angular-velocity data, a sequential Kalman filter to generate a low-frequency attitude-angle signal from the interpolated angular-velocity data and the attitude-angle calibration data, a third extractor for extracting high-frequency attitude-angle data as time series data from the telemetry data memory to generate a high-frequency attitude-angle signal, and an adder for adding the low-frequency attitude-angle signal and the high-frequency attitude-angle signal together to generate a broad-band attitude-angle signal.
The present invention also provides a method for detecting attitude of an artificial satellite, including the steps of: storing telemetry data received from the artificial satellite, extracting attitude-angle calibration data from the telemetry data memory as time-series data, extracting angular-velocity data from the telemetry data memory as time-series data, interpolating the angular-velocity data to generate interpolated angular-velocity data, generating a low-frequency attitude-angle signal from the interpolated angular-velocity data and the attitude-angle calibration data, extracting high-frequency attitude-angle data as time series data from the telemetry data memory to generate a high-frequency attitude-angle signal, and adding the low-frequency attitude-angle signal and the high-frequency attitude-angle signal together to generate a broad-band attitude-angle is signal.
In the attitude detection system of the present invention, the sequential Kalman filter has a shorter step interval in the integration, which corresponds to the data interval of the interpolated angular-velocity data in the interpolated angular-velocity signal and is thus shorter than the sampling interval of the original angular-velocity signal. Thus, the integrated data obtained by the sequential Kalman filter has a higher accuracy compared to that obtained in the proposed system. That is, this configuration provides a higher accuracy in the low-frequency attitude-angle signal. Although high-frequency noise is increased by the shorter step interval in the integration and errors of the interpolated attitude-angle signal are caused by the interpolation in the low-frequency attitude-angle signal are, these noise and the errors are cancelled by addition of the high-frequency attitude-angle signal to the low-frequency attitude-angle signal. The method of the present invention also achieves a similar advantage.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.